The present invention generally relates to a fluid pressure control device for adjusting fluid pressure in fluid pressure apparatuses mounted on a motor vehicle, for example, a steering apparatus, a suspension apparatus and a brake apparatus and more particularly, to a brake fluid pressure control device such as a brake fluid pressure control device for traction control, in which control performance is improved by employing a flow control valve without incurring rise of production cost of the brake fluid pressure control device.
Conventionally, various flow control valves suitable for a brake fluid pressure control device have been developed. For example, the assignee assigned by the present inventors proposed a flow control valve of this kind in Japanese Patent Laid-Open Publication Nos. 3-223578 (1991) and 3-234987 (1991). In this known flow control valve, for example, a spool including an orifice accommodated slidably in a sleeve is urged by an electromagnet. Pressure difference between opposite sides of the orifice, which are communicated with an input port and an output port, respectively, is determined by urging force of this electromagnet and specifies flow rate at the orifice. Therefore, in this known flow control valve, when quantity of electric current supplied to the electromagnet is determined, flow rate of working fluid flowing from the inlet 1 port to the outlet port is determined. In other words, one-to-one correspondence exists between quantity of electric current supplied to the electromagnet and flow rate of working fluid.
As compared with a so-called "ON/OFF" type solenoid selector valve in which only a fully open operation and a fully closed operation are performed, this known flow control valve is advantageous in that since flow rate of working fluid can be controlled continuously by adjusting quantity of electric current supplied to the electromagnet, control performance can be improved and that operational noises are suppressed.
Meanwhile, U.S. Pat. No. 5,261,731 discloses a valve in which pressure difference between an inlet port and an outlet port can be adjusted continuously. This prior art valve includes a valve body establishing and cutting off communication between the inlet port and the outlet port and an electromagnet for urging the valve body. Thus, in this prior art valve, by establishing and cutting off communication between the inlet port and the outlet port through change of urging force of the electromagnet, pressure difference between inlet port and the outlet port can be controlled. However, in this prior art valve, when detection members for detecting pressures of the inlet port and the outlet port, respectively are not provided, quantity of electric current supplied to the electromagnet cannot be accurately set to a predetermined value for obtaining a desired pressure difference between the inlet port and the outlet port. On the contrary, in the known flow control valve referred to above, even when pressure difference between the inlet port and the outlet port is not detected, flow rate of working fluid is determined if quantity of electric current supplied to the electromagnet is determined. Thus, the above mentioned known flow control valve is more excellent in controllability than the above described prior art valve capable of continuously adjusting pressure difference between the inlet port and the outlet port.
However, in the known flow control valve, a current drive circuit for making electric current supplied to the electromagnet variable is required to be provided and is more expensive than a general voltage drive circuit for driving the solenoid selector valve. Therefore, if both an inlet valve and an outlet valve of the fluid pressure control device are each formed by the known flow control valve, production cost of the fluid pressure control device as a whole rises undesirably.